Refactor email templates

theme/templates/email/trades/trade_update_accepted.txt

@@ -0,0 +1,15 @@
+{% include 'email/common/header.txt' %}
+
+Great news! {{ acting_user }} ({{ acting_user_ign }} {{ acting_user_friend_code }}) has accepted your trade offer.
+
+Trade Details:
+- They have: {{ want_card }}
+- They want: {{ has_card }}
+(#{{ hash }})
+
+What's next? You can now mark the trade as "Sent" once you've offered the card to them in the app, or reject the trade if needed.
+
+Visit your dashboard to manage this trade:
+{{ domain }}{% url 'dashboard' %}
+
+{% include 'email/common/footer.txt' %}